Embedded SRAM and Methods of Forming the Same

ABSTRACT

A chip includes a semiconductor substrate, and a first N-type Metal Oxide Semiconductor Field Effect Transistor (NMOSFET) at a surface of the semiconductor substrate. The first NMOSFET includes a gate stack over the semiconductor substrate, a source/drain region adjacent to the gate stack, and a dislocation plane having a portion in the source/drain region. The chip further includes a second NMOSFET at the surface of the semiconductor substrate, wherein the second NMOSFET is free from dislocation planes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/390,166, entitled “Embedded SRAM and Methods of Forming the Same,”filed Apr. 22, 2019, which is a continuation of U.S. patent applicationSer. No. 15/790,886, entitled “Embedded SRAM and Methods of Forming theSame,” filed Oct. 23, 2017 (now U.S. Pat. No. 10,269,810, issued Apr.23, 2019), which is a continuation of U.S. patent application Ser. No.15/046,150, entitled “Embedded SRAM and Methods of Forming the Same,”filed Feb. 17, 2016 (now U.S. Pat. No. 9,812,459, issued Nov. 7, 2017),which is a divisional of U.S. patent application Ser. No. 13/922,097,entitled “Embedded SRAM and Method of Forming the Same,” filed Jun. 19,2013, (now U.S. Pat. No. 9,293,466, issued Mar. 22, 2016), whichapplications are hereby incorporated herein by reference.

BACKGROUND

Static Random Access Memory (SRAM) is commonly used in integratedcircuits. SRAM cells have the advantageous feature of holding datawithout a need for refreshing. With the increasing demanding requirementto the speed of integrated circuits, the read speed and write speed ofSRAM cells also become more important. Furthermore, enough read marginand write margins are required to achieve reliable read and writeoperations, respectively. With the increasingly scaling down of thealready very small SRAM cells, however, such request becomesincreasingly demanding.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the embodiments, and the advantagesthereof, reference is now made to the following descriptions taken inconjunction with the accompanying drawings, in which:

FIGS. 1 through 15 are cross-sectional views of intermediate stages inthe manufacturing of a first Fin Field-Effect Transistor (FinFET) withdislocation planes and a second FinFET without dislocation planes on asame chip in accordance with some exemplary embodiments;

FIG. 16 illustrates a circuit diagram of a Static Random Access Memory(SRAM) cell in accordance with exemplary embodiments;

FIG. 17 is a circuit diagram of a two-port SRAM cell in accordance withexemplary embodiments;

FIG. 18 illustrates a layout of a planar transistor in accordance withsome alternative exemplary embodiments;

FIG. 19 illustrates a layout of a multi-fin FinFET in accordance withsome alternative exemplary embodiments;

FIG. 20 illustrates a layout of a single-fin FinFET in accordance withsome alternative exemplary embodiments;

FIGS. 21A and 21B illustrate the top view and the perspective view of amulti-fin FinFET with some alternative exemplary embodiments;

FIG. 22 illustrates the cross-sectional view of a multi-fin FinFET withdislocation planes and a single-fin FinFET without dislocation planes inaccordance with some exemplary embodiments; and

FIG. 23 illustrates the cross-sectional view of a first FinFET withdislocation planes and a second FinFET without dislocation planes inaccordance with some exemplary embodiments, wherein the top ends of thedislocation planes in the first FinFET is lower than the respectivesilicide regions.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the embodiments of the disclosure are discussedin detail below. It should be appreciated, however, that the embodimentsprovide many applicable concepts that can be embodied in a wide varietyof specific contexts. The specific embodiments discussed areillustrative, and do not limit the scope of the disclosure.

Transistors with multiple threshold voltages and the method of formingthe same are provided in accordance with various exemplary embodiments.The intermediate stages of forming the transistors are illustrated. Thevariations of the embodiments are discussed. Throughout the variousviews and illustrative embodiments, like reference numbers are used todesignate like elements.

FIGS. 1 through 15 are cross-sectional views and perspective views ofintermediate stages in the manufacturing of FinFETs 100′ and 200′ (FIG.15) in accordance with some exemplary embodiments. FIG. 1 illustrates aperspective view of substrate 20, which is a part of chip 2 in a wafer.Substrate 20 may be a semiconductor substrate, which may further be asilicon substrate, a silicon carbon substrate, or a substrate formed ofother semiconductor materials. Substrate 20 may be lightly doped with ap-type or an n-type impurity. Substrate 20 includes a first portion inregion 100, and a second portion in region 200. Although the portion ofsubstrate 20 between the first portion and the second portion ofsubstrate 20 is not shown in some figures, the first and the secondportions belong to a continuous substrate 20 and the same chip 2. In thesubsequently discussed examples, FinFETs 100′ and 200′ (FIG. 15) aren-type FinFETs. The teaching provided in the present disclosure,however, is readily applicable for the formation of p-type FinFETs (andplanar n-type and p-type transistors), with the conductivity types ofthe respective well regions, source and drain regions, etc. inverted.

Regions 100 and 200 may be of different types, and are referred to inaccordance with the types of devices formed therein. In someembodiments, region 100 is a logic device region for forming logictransistors therein. The logic device region does not include any memoryarray therein, and may be, or may not be, in the peripheral region ofSRAM arrays. For example, the logic device may be in the driver circuitor the decoder circuit of the SRAM arrays. Region 200 is a Static RandomAccess Memory (SRAM) region, in which SRAM cells and transistors areformed. Furthermore, region 200 may include both PMOS and NMOS devicestherein, and hence FinFET 200′ (FIG. 15) may represent a SRAM NMOSdevice and/or a PMOS device. In alternative embodiments, region 100 is amulti-fin FinFET region and region 200 is a single-fin FinFET region,with the FinFET in region 100 comprising a plurality of fins, while theFinFETs in region 200 may be single-fin FinFETs with each having asingle fin. In yet alternative embodiments, region 100 is a read-porttransistor region of a two-port SRAM cell, and region 200 is awrite-port transistor region of the same two-port SRAM cell.Furthermore, regions 100 and 200 may be planar devices regions includingplanar transistors or FinFET regions in some embodiments.

Next, referring to FIG. 2, isolation regions 22 are formed, which extendfrom a top surface of substrate 20 into substrate 20. Isolation regions22 may be Shallow Trench Isolation (STI) regions, and are referred to asSTI region 22 hereinafter. The formation of STI regions 22 may includeetching semiconductor substrate 20 to form trenches (not shown), andfilling the trenches with a dielectric material to form STI regions 22.STI regions 22 may comprise silicon oxide, for example, although otherdielectric materials may also be used. The portions of substrate 20between neighboring STI regions 22 are referred to as semiconductorstrips 124 and 224 throughout the description. Semiconductor strips 124and 224 are in regions 100 and 200, respectively. The top surfaces ofsemiconductor strips 124 and 224 and the top surfaces of STI regions 22may be substantially level with each other, although they may be atslightly different levels.

In accordance with some exemplary embodiments, the steps shown in FIG. 3and 4 are performed to replace the materials of semiconductor strips 124and 224 in order to form semiconductor strips 128 and 228. Inalternative embodiments, the replacement steps are not performed.Referring to FIG. 3, at least top portions of, or substantiallyentireties of, semiconductor strips 124 and 224 in FIG. 2 are removed.Accordingly, recesses 126 and 226 are formed between STI regions 22. Thebottom surfaces of recesses 126 and 226 may be level with the bottomsurfaces of STI regions 22. Alternatively, the bottom surfaces ofrecesses 126 and 226 are higher than or lower than the bottom surfacesof STI regions 22.

An epitaxy is performed to grow a semiconductor material in recesses 126and 226. The resulting structure is shown in FIG. 4, wherein the epitaxysemiconductor forms semiconductor strips 128 and 228 in regions 100 and200, respectively. A Chemical Mechanical Polish (CMP) is then performedto level the top surfaces of semiconductor strips 128 and 228 with thetop surfaces of STI regions 22. Semiconductor strips 128 and 228 mayhave a lattice constant greater than, substantially equal to, or smallerthan, the lattice constant of substrate 20. Furthermore, semiconductorstrips 128 and 228 may comprise silicon germanium, silicon carbon, pureor substantially pure germanium, a III-V compound semiconductor, a II-VIcompound semiconductor, or the like. For example, the availablematerials for forming III-V compound semiconductor strips 128 and 228include, but are not limited to, InAs, AlAs, GaAs, InP, GaN, InGaAs,InAlAs, GaSb, AlSb, AlP, GaP, and the like.

In some embodiments, after the epitaxy and the CMP, an implantation step25 is performed, which step is referred to as a well doping step. As aresult, well regions 127 and 227 are formed in regions 100 and 200,respectively. The bottom surfaces of well regions 127 and 227 may belower than, although they may also be level with or higher than, thebottom surfaces of STI regions 22. In some embodiments, the well dopingis performed by implanting a p-type impurity, such as boron, indium, orthe like. The dosage for implanting well regions 127 and 227 may bebetween about 1E12/cm² and about 5E14/cm², for example. In alternativeembodiments, semiconductor strips 128 and 228 are in-situ doped duringthe epitaxy to receive the well doping.

In alternative embodiments, the process steps in FIGS. 3 and 4 areskipped, and semiconductor strips 124 and 224 in FIG. 2 remain notreplaced. In which embodiments, semiconductor strips 124 and 224 in FIG.2 are also referred to as semiconductor strips 128 and 228,respectively, in subsequent discussion. Semiconductor strips 128 and 228in accordance with these embodiments are formed of the samesemiconductor material as semiconductor substrate 20. Furthermore, inthese embodiments, implantation step 25 is also performed to form wellregions 127 and 227.

Referring to FIG. 5, STI regions 22 are recessed, for example, throughan etching step. The top surfaces 22A of the remaining STI regions 22are thus lower than top surface 128A and 228A of semiconductor strips128 and 228, respectively. Throughout the description, the portions ofsemiconductor strips 128 and 228 over top surface 22A are referred to assemiconductor fins 130 and 230, respectively. Semiconductor fins 130 and230 are also referred to as the active regions for forming the resultingFinFETs.

In accordance with some embodiments, an additional p-type implantationstep 29 is performed, which may include tilt implantations from theopposite sides of semiconductor fins 130 and 230. As a result,Anti-Punch-Through (APT) regions 131 and 231 are formed. APT regions 131and 231 have higher p-type impurity concentrations than the lowerportions of semiconductor strips 128 and 228. APT regions 131 and 231are in semiconductor strips 128 and 228, and extend to the positionsslightly lower than the top surfaces of STI regions 22. In alternativeembodiments, the formation of APT regions 131 and 231 is skipped. Forclarity, in subsequent drawings, APT regions 131 and 231 are notillustrated.

Referring to FIG. 6, dummy gate stacks 132 and 232 are formed. In someembodiments, dummy gate stacks 132 and 232 include dummy gate electrodes135 and 235 and the underlying dummy gate dielectrics 133 and 233. Dummygate electrodes 135 and 235 may comprise, for example, polysilicon. Theformation of dummy gate stacks 132 and 232 may include forming blanklayer(s), performing a CMP to level the top surface of the blanklayer(s), and patterning the blank layers. The remaining portions of theblank layers(s) are dummy gate stacks 132 and 232. Dummy gate stacks 132and 232 cover middle portions 130B and 230B of semiconductor fins 130and 230, respectively. Opposite end portions 130A and 230A ofsemiconductor fins 130 and 230 are not covered. Dummy gate stacks 132and 232 may also have a lengthwise direction substantially perpendicularto the lengthwise direction of semiconductor fins 130 and 230. Althoughnot shown in FIG. 6, gate spacers 137 and 237 (FIG. 7) are also formedon the sidewalls of dummy gate stacks 132 and 232.

FIGS. 7 through 12 illustrate the cross-sectional views in the formationof source and drain regions and dislocation planes in regions 100 and200. The cross-sectional views are obtained from the planes crossinglines A-A in FIG. 6. Referring to FIG. 7, implant blocking layer 239 isformed to cover the structures in region 200, while the structures inregion 100 are not covered. Next, the portion of substrate 20 in region100 is recessed, forming recesses 140. In some embodiments, therecessing is anisotropic, so that the sidewalls of recesses 140 aresubstantially vertical, and are aligned to the sidewalls of gate spacers137, although recesses 140 may also extend underlying gate spacers 137.Although recesses 140 are illustrated as having rectangularcross-sectional shapes, they may also have other shapes such asspade-shapes. In the recessing of substrate 20 in region 100, theportions of substrate 20 in region 200 are protected by implant blockinglayer 239.

Next, an amorphization implantation 142 is performed, forming amorphizedregions 144, which are portions of the substrate 20 under recesses 140.In some embodiments, the amorphization implantation 142 is performed byimplanting substrate 20 with species such as germanium, silicon, or thelike. As a result of amorphization implantation 142, as shown in FIG. 7,amorphized regions 144 are formed, which may include amorphized siliconin some embodiments. The portions of substrate 20 in region 200 areprotected from the implantation by implant blocking layer 139, andremain to have a crystalline structure.

Next, as shown in FIG. 8, strained capping layer 46 is formed on thestructure shown in FIG. 7. Strained capping layer 46 is formed in deviceregion 100, and may, or may not, extend into region 200. The materialsof strained capping layer 46 may include silicon nitride, titaniumnitride, oxynitride, oxide, SiGe, SiC, SiON, and/or combinationsthereof. Strained capping layer 46 may have an inherent tensile stress.The formation process of strained capping layer 46 is adjusted to tunethe stress of strained capping layer 46 to a desirable value.

An annealing (represented by arrows 48) is then performed to formdislocation planes 150 in amorphized regions 144 (FIG. 7). The annealingmay be performed using Rapid Thermal Anneal (RTA), laser anneal, orother anneal methods. In some embodiments, the annealing is performedusing spike RTA, with the annealing temperature between about 900° C.and about 1100° C., for example. As a result of the annealing,amorphized regions 144 as in FIG. 7 are recrystallized to formcrystalline regions 145, with a memorized stress obtained from strainedcapping layer 46.

As the result of the annealing, dislocation planes 150 are also formedin crystalline regions 145. Although illustrated as lines in thecross-sectional view as shown in FIG. 8, dislocation planes 150 areplanes that extend parallel to the longitudinal direction of dummyelectrode 135. In the crystalline regions 145 that are between two dummygate electrodes 135, there may be two dislocation planes 150 tilting inopposite directions. In a crystalline region 146 that is formed betweena dummy gate electrode 135 and a neighboring STI 22, there may be asingle dislocation plane 150 (marked as 150A) formed. The otherdislocation plane 150B may be formed, or may not be formed if the widthof the crystalline regions 145 is not great enough. Furthermore, inregion 200, no dislocation plane is formed.

Next, implant blocking layer 139 and strained layer 46 are removed, andthe resulting structure is shown in FIG. 9. In a subsequent step,referring to FIG. 10, recesses 240 are formed in region 200. In theformation of recesses 240, the structure in region 100 may be protectedby a mask layer (not shown), which is removed after the formation ofrecesses 240. In a subsequent step, a shown in FIG. 11, source and drainregions (denoted as source/drain regions hereinafter) 152 and 252 areformed through epitaxy. The top view and the perspective view of sourceand drain regions 152/252 are shown in FIGS. 21A and 21B, respectively.As shown in FIG. 11, the source/drain regions 152/252 betweenneighboring gate stacks 132 are common sources or common drains.Furthermore, as shown in FIG. 21B, the neighboring source regions maymerge with each other, and the neighboring drain regions may merge witheach other. During the epitaxy, SiP, SiC, SiPC, silicon, or the like maybe epitaxially grown. Furthermore, an n-type impurity such asphosphorous, arsenic, or the like, may also be in-situ doped with theproceeding of the epitaxy for forming n-type FinFETs. In the embodimentsthe devices in region 200 comprise a p-type FinFET, a p-type impuritysuch as boron may be in-situ doped. In these embodiments, however, theepitaxy regions in regions 100 and 200 are formed separately.

During the epitaxy, dislocation planes 150 also grow with the proceedingof the epitaxy, and hence dislocation planes 150 are also formed insource/drain regions 152. On the other hand, no dislocation plane isgrown in source/drain regions 252. FIG. 12 illustrates the formation ofsource/drain silicides regions 154 and 254.

FIG. 13 illustrates a perspective view of the structure afterInter-Layer Dielectric (ILD) 56 is formed. ILD 56 comprises a dielectricmaterial such as Phospho-Silicate Glass (PSG), Boro-Silicate Glass(BSG), Boron-Doped Phospho-Silicate Glass (BPSG), or the like. A CMP maybe performed to level the top surface of ILD 56 with the top surface ofdummy gate stacks 132 and 232. Accordingly, source/drain regions 152 and252 are buried under ILD 56. Although not shown, before the formation ofILD 56, spacers may be formed on the opposite sidewalls of dummy gatestacks 132 and 232, wherein the spacers may be formed of a materialdifferent from the materials of ILD 56 and dummy gate stacks 132 and232.

Next, dummy gate stacks 132 and 232 are removed in an etching step, sothat trenches 157 and 257 are formed in ILD 56. The resulting structureis shown in FIG. 14. Trenches 157 and 257 are located in regions 100 and200, respectively. Replacement gates are then formed, as shown in FIG.15, which illustrates cross-sectional views of regions 100 and 200. TheCross-sectional views in FIG. 15 are retrieved from the same planes thatcross lines B-B and C-C in FIG. 14. The replacement gates include gatedielectrics 158 and 258 and gate electrodes 160 and 260.

The intermediate stages in the formation of gate dielectrics 158 and 258and gate electrodes 160 and 260 are not illustrated, and are describedbriefly below. In the formation process, a gate dielectric layer (notshown) is formed as a blanket layer in trenches 157 and 257 (FIG. 14)and on the top surfaces and the sidewalls of semiconductor fin portions130B and 230B and ILD 56. In accordance with some embodiments, the gatedielectric layer comprises silicon oxide, silicon nitride, ormultilayers thereof. In alternative embodiments, the gate dielectriclayer comprises a high-k dielectric material. In which embodiments, thegate dielectric layer may have a k value greater than about 7.0, and mayinclude a metal oxide of Hf, Al, Zr, La, Mg, Ba, Ti, Pb, andcombinations thereof. Next, a conductive material (not shown) isdeposited over the gate dielectric layer, and fills the remainingtrenches 157 and 257 (FIG. 14). The conductive material may comprise ametal-containing material such as TiN, TaN, TaC, Co, Ru, Al,combinations thereof, or multi-layers thereof. After the deposition ofthe conductive material, a CMP is performed to remove the excessportions of the gate dielectric layer and the conductive material, whichexcess portions are over the top surface of ILD 56. The resultingremaining portions of the conductive material and the gate dielectriclayer thus form the replacement gates of the resulting FinFETs 100′ and200′ in regions 100 and 200, respectively.

FIG. 16 illustrates a circuit diagram of SRAM cell 300 in accordancewith some embodiments. SRAM cell 300 includes pass-gate transistors PG-1and PG-2 and pull-down transistors PD-1 and PD-2, which are N-typeMetal-Oxide-Semiconductor (NMOS) transistors. SRAM cell 300 furtherincludes pull-up transistors PU-1 and PU-2, which are P-typeMetal-Oxide-Semiconductor (PMOS) transistors, The gates of pass-gatetransistors PG-1 and PG-2 are connected to word-line WL that determineswhether SRAM cell 300 is selected or not. A latch formed of pull-uptransistors PU-1 and PU-2 and pull-down transistors PD-1 and PD-2 storesa bit, wherein the complementary values of the bit are stored in datanode A and data node B. The stored bit can be written into, or readfrom, SRAM cell 300 through bit lines BL and BLB.

The sources of pull-up transistors PU-1 and PU-2 are connected tovoltage node Vdd, which carries positive power supply voltage (and line)Vdd. The sources of pull-down transistors PD-1 and PD-2 are connected topower supply node Vss, which are further connected to power supplyvoltage/line Vss (an electrical ground, for example). The gates oftransistors PU-1 and PD-1 are connected to the drains of transistorsPU-2 and PD-2, which connection node is data node A. The gates oftransistors PU-2 and PD-2 are connected to the drains of transistorsPU-1 and PD-1, which connection node is data node B. A source or drainregion (referred to as source/drain region hereinafter) of pass-gatetransistor PG-1 is connected to bit-line BL. A source/drain region ofpass-gate transistor PG-2 is connected to bit-line BLB.

In some embodiments, transistors PD-1 and PD-2 and/or transistors PG-1and PG-2 are formed using essentially the same process as for formingdevice 200′ in region 200 (FIG. 15). Transistors PD-1 and PD-2 and/ortransistors PG-1 and PG-2 thus do not have dislocation planes formedtherein. Without the dislocation planes, the threshold voltages oftransistors PD-1 and PD-2 and/or transistors PG-1 and PG-2 are high thanif dislocation planes are formed. With higher threshold voltages, thestates of SRAM cell 300 are more stable. On the other hand, in the samechip and on the same substrate 20, a device 100′ (FIG. 15) is alsoformed. Device 100′, with the dislocation planes, has a lower thresholdvoltage and hence a higher on-current than device 200′, and can be usedas, for example, a logic device due to its higher performance. Hence,the present disclosure provides a multi-Vt solution for formingtransistors having different threshold voltages to suit to differentrequirements.

FIG. 17 illustrates a circuit diagram of two-port SRAM cell 400, whichincludes a write port and a read port. The write port includestransistors PU-1 and PD-1, and FinFETs PU-2 and PD-2. The write portfurther includes pass-gate transistors W_PG-1 and W_PG-2, wherein thegates of transistors W_PG-1 and W_PG-2 are coupled to write word-lineW-WL. The writing of SRAM cell 400 is through complementary writebit-lines W-BL and W-BLB. The read port includes transistors PU-1 andPD-1, transistors PU-2 and PD-2, pull-down transistor R-PD, andpass-gate transistor R-PG. The data retrieved from SRAM cell is sent toread bit-line R-BL. Transistors R-PD is further coupled to positivepower supply CVdd. Transistors R-PD and R-PG are cascaded. The gate oftransistor RPG may be coupled to read word-line R-WL.

In some embodiments, transistors PD-1 and PD-2 (and possibly W_PG-1 andW_PG-2) are formed in device region 200 (FIG. 15) and having thestructures of device 200′. Therefore, they have high threshold voltages,and hence SRAM cell 400 is highly stable. Transistors R-PD and RPG, onthe other hand, do not have the stability concern. Hence, they areformed in device region 100 (FIG. 15) and have the structures of device100′, so that they can have lower threshold voltages and higheron-currents. The read speed of the read port is hence improved withoutsacrificing the stability of SRAM cell 400. Therefore, the presentdisclosure provides a solution of incorporating multi-threshold voltagedevices in a same SRAM cell without increasing the manufacturing cost.

FIG. 18 illustrates a layout of a planar transistor 500, which includesactive region 502, gate electrodes 504, and source/drain contacts 506.In accordance with some embodiments, the structure of the planar devicein FIG. 18 may be used to form a first planar transistor havingdislocation planes, and a second planar transistor without dislocationplanes, with the first and the second planar transistors being on thesame substrate and in the same chip. The planar devices may also be usedfor forming the SRAM cell device and the logic device. For example, theNMOS transistors PD-1 and PD-2 in SRAM cell 300 may be formed using theplanar transistor that do not have dislocation planes, while a logictransistor (or any other transistor requiring a higher on-current) maybe formed using the planar transistor that has dislocation planes.

FIGS. 19 and 20 illustrate the layout of a multi-fin FinFET 600 and asingle-fin FinFET 700, respectively. In FIG. 19, a plurality ofsemiconductor fins 130 (also refer to FIG. 5) are formed, and gateelectrodes 160 are formed over semiconductor fins 130. In FIG. 20, asingle semiconductor fin 230 (also refer to FIG. 5) is formed, and gateelectrodes 260 are formed over semiconductor fin 230. Thecross-sectional views of multi-fin FinFET 600 and single-Fin FinFET 700are illustrated in FIG. 22. In these embodiments, multi-fin FinFET 600has dislocation planes 150, while single-fin FinFET 700 does not havedislocation planes 150 formed therein. Accordingly, in some exemplaryembodiments, multi-fin FinFET 600, due to the multiple fins and lowerthreshold voltage caused by not forming the dislocation planes, has avery high on-current, and can be used as, for example, logic device 100′ in FIG. 22. On the other hand, single-fin FinFET 700 has a highthreshold voltage, and can act as devices PD-1 and PD-2 (and/or PG-1 andPG-2) (FIG. 16) in SRAM cell 400. Alternatively, the single-fin FinFET700 that have no dislocation planes may form the write-port devices PD-1and PD-2 (and/or W_PG-1 and W_PG-2) in a two-port SRAM cell (FIG. 17),while the multi-fin FinFETs 600 that have dislocation planes may formthe read-port devices R-PD and R-PG. The formation of the structure inFIG. 22 is essentially the same as in FIGS. 1 through 15.

FIG. 23 illustrates a structure in accordance with alternativeembodiments. These embodiments are similar to the embodiments in FIG.15, except that dislocation planes 150 tilting in different directionsmet with each other, and end at a point lower than the bottom surface ofsilicide regions 154. Distance Di between the end point of dislocationplanes 150 and the bottom surface of silicide regions 154 may be betweenabout 3 nm and about 10 nm, for example.

In the embodiments of the present disclosure, transistors havingdifferent threshold voltages are formed by forming dislocation planesselectively, and/or by selecting the number of fins. The differencebetween the threshold voltages of the devices that have or don't havedislocation planes may be greater than about 40 mV, and may be betweenabout 40 mV and about 100 mV. The requirement of different circuits maythus be met without significantly increasing manufacturing cost.

In accordance with some embodiments, a chip includes a semiconductorsubstrate, and a first NMOSFET at a surface of the semiconductorsubstrate. The first NMOSFET includes a gate stack over thesemiconductor substrate, a source/drain region adjacent to the gatestack, and a dislocation plane having a portion in the source/drainregion. The chip further includes a second NMOSFET at the surface of thesemiconductor substrate, wherein the second NMOSFET is free fromdislocation planes.

In accordance with other embodiments, a chip includes a multi-fin n-typeFinFET and a single-fin n-type FinFET. The multi-fin n-type FinFETincludes a plurality of semiconductor fins, a first gate stack onsidewalls and top surfaces of the plurality of semiconductor fins, afirst source/drain region adjacent to the first gate stack, and adislocation plane having a portion in the first source/drain region. Thesingle-fin n-type FinFET includes a single semiconductor fin, a secondgate stack on a sidewall and a top surface of the single semiconductorfin, and a second source/drain region adjacent to the first gate stack,wherein no dislocation plane extends into the second source/drainregion.

In accordance with yet other embodiments, a method includes forming afirst gate stack in a first device region and a second gate stack in asecond device region, and forming an implant blocking layer to cover thesecond device region, wherein the first device region is not covered bythe implant blocking layer. The method further includes recessing aportion of a semiconductor region in the first device region to form arecess, and performing an amorphization implantation to form anamorphized region under the recess, wherein no amorphized region isformed in the second device region. A strained capping layer is thenformed over the first gate stack and the amorphized region, followed byan annealing to re-crystalize the amorphized region to form are-crystallized region, wherein a dislocation plane is formed in there-crystallized region. The method further includes removing thestrained capping layer and the implant blocking layer, epitaxiallygrowing a first source/drain region in the recess, and forming a secondsource/drain region in the second device region and adjacent to thesecond gate stack.

Although the embodiments and their advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the embodiments as defined by the appended claims. Moreover,the scope of the present application is not intended to be limited tothe particular embodiments of the process, machine, manufacture, andcomposition of matter, means, methods and steps described in thespecification. As one of ordinary skill in the art will readilyappreciate from the disclosure, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed, that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the disclosure.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps. In addition, each claim constitutes a separateembodiment, and the combination of various claims and embodiments arewithin the scope of the disclosure.

What is claimed is:
 1. A structure comprising: an SRAM cell comprising aplurality of first NMOS transistors and a plurality of PMOS transistors,each of the plurality of first NMOS transistors comprising: a first finover a substrate; a first gate structure over the first fin; and firstsource/drain regions on opposing sides of the first gate structure,wherein the first source/drain regions are free of dislocations; and alogic circuit electrically coupled to the SRAM cell, the logic circuitcomprising a second NMOS transistor, the second NMOS transistorcomprising: a second fin over the substrate; a second gate structureover the second fin; and second source/drain regions on opposing sidesof the second gate structure, wherein each of the second source/drainregions comprises a first dislocation plane extending parallel to alongitudinal axis of the second fin.
 2. The structure of claim 1,wherein at least one of the second source/drain regions of the secondNMOS transistor comprises a second dislocation plane.
 3. The structureof claim 2, wherein the first dislocation plane and the seconddislocation plane merge.
 4. The structure of claim 2, wherein the firstdislocation plane and the second dislocation plane do not merge.
 5. Thestructure of claim 4 further comprising a silicide region on the atleast one of the second source/drain regions of the second NMOStransistor, wherein the first dislocation plane and the seconddislocation plane terminate below a bottom surface of the silicideregion.
 6. The structure of claim 5, wherein the first dislocation planeand the second dislocation plane are spaced apart from the bottomsurface of the silicide region by a distance between 3 nm and 10 nm. 7.The structure of claim 1, wherein the second NMOS transistor comprises athird fin, the second gate structure extending over the third fin,wherein each of the second source/drain regions comprises a continuousepitaxial region between the second fin and the third fin.
 8. Astructure comprising: a first transistor comprising: a first fin over asubstrate; a first gate structure over the first fin; and firstsource/drain regions on opposing sides of the first gate structure,wherein the first source/drain regions comprise an epitaxial material,the first source/drain regions are free of dislocations; and a secondtransistor comprising: a second fin over the substrate; a second gatestructure over the second fin; and second source/drain regions onopposing sides of the second gate structure, wherein the secondsource/drain regions comprise the epitaxial material, wherein each ofthe second source/drain regions comprises a first dislocation plane,wherein the first dislocation plane is parallel to a longitudinal axisof the second fin.
 9. The structure of claim 8 further comprising anSRAM memory cell, wherein the SRAM memory cell comprises the firsttransistor.
 10. The structure of claim 9, wherein the second transistoris part of circuitry electrically coupled to the SRAM memory cell. 11.The structure of claim 8, wherein the second source/drain regionscomprise a first epitaxial region and a second epitaxial region, whereinthe first dislocation plane in the first epitaxial region is an onlydislocation plane in the first epitaxial region.
 12. The structure ofclaim ii, wherein the second epitaxial region comprises a seconddislocation plane.
 13. The structure of claim 12 further comprising: anisolation region, wherein the first epitaxial region is adjacent theisolation region.
 14. The structure of claim 12, wherein the firstdislocation plane and the second dislocation plane extend toward eachother as the first dislocation plane and the second dislocation planeextends upward.
 15. The structure of claim 8, wherein a differencebetween a first threshold voltage of the first transistor and a secondthreshold voltage of the second transistor is between 40 mV and 100 mV.16. A structure comprising: a first transistor comprising: a first finover a substrate; a first gate structure over the first fin; a secondgate structure over the first fin; and a common source/drain regioninterposed between the first gate structure and the second gatestructure, wherein the common source/drain region comprises an epitaxiallayer over the substrate, the epitaxial layer comprising a semiconductormaterial different than the substrate, the epitaxial layer comprising afirst dislocation plane and a second dislocation plane, wherein thefirst dislocation plane and the second dislocation plane extend upwardtoward each other, the first dislocation plane not intersecting thesecond dislocation plane.
 17. The structure of claim 16 furthercomprising a silicide region over the common source/drain region,wherein a region of the epitaxial layer being free of the firstdislocation plane and the second dislocation plane is interposed betweenthe silicide region and ends of the first dislocation plane and thesecond dislocation plane.
 18. The structure of claim 17, wherein adistance between an end of the first dislocation plane and the silicideregion is between 3 nm and 10 nm.
 19. The structure of claim 16 furthercomprising: an isolation region; and another source/drain regioninterposed between the first gate structure and the isolation region,wherein another source/drain region has a single dislocation plane. 20.The structure of claim 16, wherein the first dislocation plane extendsunder the first gate structure.